1. Field of Invention
The invention relates to an image forming apparatus, and more specifically, to an arrangement of a fixing device for an image forming apparatus, such as an electrophotographic copier, an electrophotographic printer (i.e., a laser printer, an LED printer), a facsimile machine, and a word processor.
2. Description of Related Art
Conventionally, an image forming apparatus includes a fixing device to fix a toner image transferred onto a printing medium. The fixing device includes a heat roller and a pressure roller, which are disposed adjacent each other at respective roller surfaces. The printing medium fed into the fixing device is sandwiched between the heat roller and the pressure roller and fed by rotation of the heat roller. At this time, the toner is fixed onto the printing medium by the application of pressures of the heat roller and the pressure roller and the application of heat of the heat roller.
A drive gear supported to the main unit of the image forming apparatus drives a heat roller gear attached coaxially with the heat roller, which rotates the heat roller. If a pitch being a distance between a center of rotation of the drive gear and a center of rotation of the heat roller gear is not fixed, the rotation of the heat roller becomes unstable, thereby deteriorating image quality.
A known method for maintaining the pitch between the drive gear and the heat roller gear at a specific value is disclosed in Japanese Laid-Open Patent Publication No. 53-135643. In this method, the drive gear is attached to a main body frame to which the fixing device is mounted. A mating part is provided in the main body frame to make contact with a mating part in the fixing device. The fixing device is screwed to the main body frame via an elongated hole with the mating parts in contact with each other, thereby securing the fixing device at a determined position with respect to the main body frame. Therefore, the distance (pitch) between the center of rotation of the heat roller gear in the fixing device and the center of rotation of the drive gear in the main body frame is maintained at the specified value.
However, in this method, the screw to tighten the fixing device to the main body frame comes loose due to use over an extended period of time and vibrations, and the fixing device is shifted from the determined position with respect to the main body frame.
As a torque is transmitted from the drive gear to the heat roller gear, the torque acts on the entire fixing device. In particular, when the torque from the drive gear is great, the torque that acts on the entire fixing device is also great. As a result, if the fixing device is not firmly secured to the main body frame, the fixing device becomes misaligned. When the fixing device is shifted from the proper installation position in the main body frame, the rotation of the heat roller becomes unstable, and the image becomes distorted during fixing, thereby deteriorating the fixed image quality.
The invention provides an improved image forming apparatus that addresses the foregoing drawbacks associated with image forming apparatus.
According to an aspect of the invention, an image forming apparatus includes a main body; a heat roller that heats toner of a toner image formed on a printing medium to fix the toner image; a heat roller gear that transmits a rotational driving force to the heat roller; a fixing device frame that supports the heat roller and the heat roller gear and is attached to the main body; and a gear supporting member that supports a drive gear that transmits the driving force in engagement with the heat roller gear and is attached to the main body. At least one of the fixing device frame and the gear supporting member is relatively moved in a direction to approach the other by the driving force the drive gear transmits, while a distance between a rotation center of the heat roller gear and a rotation center of the drive gear is maintained to a specified value.
According to another aspect of the invention, an image forming apparatus includes a main body, a heat roller that heats toner of a toner image formed on a printing medium to fix the toner image, a heat roller gear that transmits a rotational driving force to the heat roller, a fixing device frame that supports the heat roller and the heat roller gear and is attached to the main body so as to rotate on a fixing point, a gear supporting member that supports a drive gear that transmits the driving force in engagement with the heat roller gear and is attached to the main body, a frame contact part that is provided in the fixing device frame, and a gear contact part that makes contact with the frame contact part. The fixing device frame is rotated on the fixing point by the rotational driving force the drive gear applies to the heat roller and the frame contact part makes contact with the gear contact part.